The present invention relates to a process for the preparation of N-(2,3Dihydrobenzo[1,4]dioxin-2-carbonyl) piperazine.
N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl) piperazine is an important intermediate for the preparation of an anti-hypertensive drug viz., doxazocin. Doxazocin is prepared by reaction of N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine with 4-amino-2-chloro-6,7-dimethoxy quinazoline [J. Med. Chem., 1987, 30(1), 49]. However, it was observed that the product obtained by this condensation contained bis-amide as impurity. It makes the product unsuitable for pharmaceutical use as doxazocin base prepared by basification, on analysis showed purity of 91-94% only. Further purification of the base was very difficult due to its polymorphic nature. Large number of references are available mostly of patents (U.S. Pat. Nos. 6,140,334; 6,133,269; 6,130,218) in which doxazocin base has been subjected to tedious purification procedures consisting of four to five steps in order to obtain pure doxazocin of pharmaceutical grade.
Pharmaceutical industry requires pure doxazocin (99.99%) for processing into solid dosage form. In our attempts to prepare pure doxazocin, we found that the purity and crystalline nature of doxazocin were dependent on the purity of N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl) piperazine that was used for the condensation. While it was easy to prepare 4-amino-2-chloro-6,7-dimethoxy quinazoline in high yield and purity, the preparation of N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine posed several problems due to its contamination with the bis-amide impurity. It is glassy and difficult to work with. It is essential to evolve a method for the preparation of pure amide not contaminated with bis-amide,
N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine is a condensation product of ethyl 2,3-Dihydrobenzo[1,4]dioxin-2-carboxylate and piperazine [Equation 1]. 
N-Acyl alkylenediamines were earlier prepared by the monoamidation of alkylenediamines with acid chlorides. The reaction requires the masking of diamines as monoacetates or hydrogen halides [J. Med. Chem.,1997, 20, 146]. The procedure is tedious and requires i) saponification of ester ii) treatment of the resulting acid with thionyl chloride to give the acid chloride and iii) amidation with piperazine hydrobromide in methanol. There is also one report of the synthesis of N-Acyl alkylenediamnines (94% yield) by direct monoamidation of esters [J. Org. Chem., 1998, 63, 10015]. In this procedure, ethyl 2,3-dihydrobenzo[1,4]dioxin-2-carboxylate and piperazine were heated under reflux for 3 h. The reaction mixture was partitioned between CHCl3 and saturated aq. NaHCO3. The organic layer was washed, dried and concentrated. However, N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine prepared by the above method when coupled with 4-amino-2-chloro-6,7-dimethoxy quinazoline gave a product, which on subsequent basification, gave doxazocin base with 92-95% purity. In view of the above, it is necessary to develop an improved process for the preparation of high purity N(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine without contamination with bis-amide.
The main objective of the present invention is to provide a process for the preparation of high purity N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine of Formula 1.
Another object of the present invention is to provide N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine free from bis-amide and other impurities by simple acid base work-up procedure. 
Accordingly, the present invention provides an improved process for the preparation of N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine which comprises heating a reaction mixture ethyl of 2,3-dihydrobenzo[1,4]dioxin-2-carboxylate and piperazine with a molar ratio of carboxylate to piperazine in the range of 1:2 to 2:1, under nitrogen atmosphere, at a temperature in the range of 70-90xc2x0 C. for a period of 3-15 hrs, cooling the above said reaction mixture to a temperature of 25-30xc2x0 C. and dissolving it in chloroform followed by washing with saturated sodium bi-carbonate solution and water respectively, acidifying the organic layer by about 10% HCl to a pH ranging between 1 to 4, separating the organic layer and washing the aqueous layer with chloroform, basifying the aqueous layer with sodium bicarbonate to a pH ranging between 7 to 9 followed by extracting with chloroform and evaporating the chloroform to obtain the final desired product in amorphous solid form.
In an embodiment of the present invention the reaction mixture is heated for a period preferably in the range of 5-12 hrs.
In yet another embodiment the temperature used in heating reaction mixture is preferably in the range of 75-80xc2x0 C.
In still another embodiment the purity of the compound N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine obtained is in the range of 99.3-99.9%.
In the improved process for the preparation of N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl)piperazine, ethyl 2,3-dihydrobenzo[1,4]dioxin-2-carboxylate and piperazine were reacted under a blanket of nitrogen at a temperature preferably in the range of 70-90xc2x0 C. for a period preferably in the range of 5-12 h. The reaction mixture on cooling was taken into chloroform. The chloroform layer is acidified to a pH preferably in the range of 4-2. The aqueous layer was basified using solid NaHCO3 to a pH preferably in the range of 7-8. It is again extracted into chloroform, dried and evaporated to give pure N-(2,3-Dihydrobenzo[1,4]dioxin-2-carbonyl) piperazine as a white solid without any contamination. Pharmaceutical grade doxazocin is prepared with this amide in a further process.